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A dam is any retaining structure, built across a river to stop or regulate its flow, and to raise the water level. Once a dam is built, water accumulates behind it to form a lake or dam reservoir. Thus it also includes containment structures for a large or small body of impounded water.
While dams are built to stop flooding and facilitate shipping, they may also be intended to generate electricity, and these are called hydroelectric dams. Such a structure is an essential part of a hydroelectric project.
Construction
Dam building is a hard and skilled job which begins with diverting the flow of the water temporarily. Following this, the area of the river bed which acts as the site for the foundation is prepared for construction, which involves ensuring that the rocky or sedimentary bed is strong enough to support the dam. The area on either side may also be strengthened. Following this a concrete block formation is built up. Once it is high enough, the water is allowed to flow in under strict control with careful monitoring. Other operational structures are finally added.
How it Works
Dams are the high-visibility part of a hydroelectric system, storing water at a height. When this water is released it flows down by gravity, and the potential energy is converted to kinetic energy. The height of the water level and the rate of flow of water decide the electricity output. The water flow is regulated by a gate or shutter which in turn is determined by the demand for power. The water flows down a series of channels or penstocks which guide and regulate the steepness of the fall, so as to optimize the efficiency of the dam.
The water finally passes through a hydraulic turbine and then into a tailrace, or exit stream, at the bottom of the dam, to rejoin the river.
Hydroelectric power is generated by the controlled flow of water through the turbine, a device that has a shaft connected to a generator that contains metal coils surrounded by magnets. As the water spins the turbine shaft, the magnets revolve around the metal coils, leading to electromagnetic induction, a phenomenon which generates electric power. In this way potential energy in the stored water is converted to kinetic energy as it flows down the penstock, and then into mechanical energy in the rotating turbine shaft, which in turn is converted to electrical energy.
Energy demands vary over the day. One advantage of hydroelectric plants is that they can vary their output to cater to peak power demands over short periods by the incorporation of pumped storage. The water which flows through the turbine is pumped into a higher storage pool located above the turbine, using excess power generated during the period of low consumer demand for power. At peak power demand this is released into the lower stream again to generate more electricity. In this way the reservoir behaves as a type of battery.
Advantages
Hydroelectric plants can be put into operation quickly. Their power output is easily adjusted. Their operation remains efficient whether for a short or long period. Construction costs are low for small or medium-sized dams compared with other hydropower plants. A dam produces electricity without burning fossil fuels, and is thus considered a source of clean energy.
Sizes
Hydroelectric dams come in a range of sizes:
- Large - A dam which has a capacity of over 30 MW is a large project.
- Small - A hydroelectric dam with a capacity of 10 MW or less.
- Micro - A microhydroelectric dam generates up to 100 KW of power, such as for a single home, a ranch or a village.
Disadvantages
Dams do produce some negative environmental impact. For instance, many species of fish cannot migrate for spawning purposes once their rivers are blocked by dams. In the USA alone, some salmon populations have come down to a couple of million from over 15 million following dam construction.
Dams and reservoirs also cause the natural temperature, the chemical composition, the type of flow and the silt loads in the river water to change, which affects the life in the river and its banks.
Dam reservoirs spread over hundreds or millions of hectares of agricultural land or land that has been settled on by a large and productive population, or archeological sites.
The human cost of dam building includes these physical changes, resettlement, and other far-reaching consequences.
The concrete and other materials used in dam construction are products of fossil fuel burning, but this one-time environmental cost is thought to be offset by the availability of clean power over a long period of 50 to 100 years.
Sometimes reservoirs serve as a source of greenhouse gases such as carbon dioxide and methane, which can have significant impact due to the large amount produced. However, scientists have developed means to use this methane to produce more electricity, which can enhance the power output while minimizing the environmental cost.
Sources
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